1. Field of the Invention
The present invention relates to a tilt steering column for a vehicle and more specifically relates to a tilt steering column using a bending bracket as an energy absorbing means.
2. Description of the Related Art
Steering columns for vehicles can be classified into two types: a non-tilt type steering column, and a tilt type steering column in which an inclination of a steering column tube assembly with respect to a vehicle body can be adjusted.
When a driver strongly strikes a steering wheel at the time of a front end impact on a vehicle, an upper column tube which supports a steering shaft connected to the steering wheel is required to move frontward in the axial direction so as to reduce the impact force on the driver from the steering wheel and to protect the driver from injury.
A well-known means for effectively reducing the impact force is an energy absorbing bending bracket which deforms when the upper column tube moves frontward and absorbs a large amount of energy through the permanent deformation thereof. FIG. 10 shows a typical conventional bending bracket 102 which is used for a conventional non-tilt steering column 100. The energy absorbing bending bracket 102 is fixed to a column tube 104 at one end thereof and is fixed to a vehicle body 106 at the other end thereof. When a steering wheel 108 receives an excessively large frontward force P' from a driver, the energy absorbing bending bracket 102 deforms in the direction of arrow A from the position shown by a full line to the position shown by a two-dotted line in FIG. 10 and absorbs energy.
Japanese Utility Model Publication SHO No. 60-6670 discloses a typical conventional tilt steering column device which has no energy absorbing bending bracket.
FIGS. 6-9 show a hypothetical tilt steering column device with a bending bracket which might be obtained if one were to combine the tilt steering column device of SHO 60-6670 and the energy absorbing bending bracket of FIG. 10. Although the tilt steering column with an energy absorbing bending bracket of FIGS. 6-9 does not constitute prior art with respect to the present application, the tilt steering column device of FIGS. 6-9 illustrates the problems arising in the design of a conventional tilt steering column device using an energy absorbing bending bracket.
In the hypothetical tilt steering column device 110 of FIGS. 6-9, a steering column tube assembly 118 is supported by a tilt-lock means 114 which is fixed to a vehicle body 112. When the tilt-lock means 114 is unlocked by rotating a tilt lever 116 in an unlocking direction B from a position shown by a full line to a position shown by a two-dotted line in FIG. 6, the column tube 118 can tilt around a tilt center 120 in a direction C in FIG. 6. An energy absorbing bending bracket 122 extends from a first fixing portion where the bending bracket 122 is fixed to the column tube 118 to a second fixing portion where the bending bracket 122 is fixed to the tilt-lock means 114. When an upper column tube 118b moves relative to a lower column tube 118a at a time of a front impact on the vehicle, the energy absorbing bending bracket 122 absorbs energy by deforming permanently. An attachment 150 is fixed to the energy absorbing bracket 122 and a collar 126 is fixed to the attachment 150. The assembly of the energy absorbing bending bracket 122, the attachment 150 and the collar 126 is so supported by the tilt-lock means 114, via a vertically elongate hole 128 formed in the tilt-lock means 114 and a lock bolt 130 passing through the long hole 114, that the tilt-lock means 114 can be locked/unlocked by tightening/loosening the lock bolt 130 due to rotation of the tilt lever 116.
However, in the hypothetical steering column device of FIGS. 6-9, there arise the following two problems.
The first problem relates to a low rigidity of the support of the column tube assembly 118 in the vertical direction. When a driver gets into the vehicle cabin, he may hold the steering wheel to support his body and may push the steering wheel downward. When the steering wheel 132 receives such a normal downward force D as shown in FIG. 8, a bending moment M will occur around the tilt-lock means 114 and, as a result, the column tube assembly 118 often undesirably slips downward from a position shown by a full line to a position shown by a two-dotted line in FIG. 8. Further, since the column tube assembly 118 is supported in the vertical direction by the energy absorbing bending bracket 122 and the tilt-lock means 114, and since the rigidity of the energy absorbing bending bracket 122 itself is not great, the rigidity in the vertical direction of an entire steering column device including the support for the column tube assembly can not be designed to be large and thus vibration of the column tube assembly becomes a problem.
The second problem relates to a small frontward movement of the upper column tube 118b. When the steering wheel 132 receives an excessively large frontward force P shown in FIG. 6 from a driver at the time of a front impact on a vehicle, the column tube assembly 118 will also be pushed upward in the direction E shown in FIG. 9 from a position shown by a full line to a position shown by a two-dotted line in FIG. 9. As a result, interference between the column tube assembly 118 and the collar 126 will occur at a position S in FIG. 9, and such interference will prevent the upper column tube 118b from moving by a sufficient distance and will prevent the energy absorbing bending bracket 122 from deforming sufficiently to absorb a large amount of energy.